Enhancing operational stability of an ultra-high-head pump-turbine using guide-vanes with horizontal blades

Under ultra-high-head pumping operations (pump head greater than 600 m), the rolling vortices in the vaneless space of a pump-turbine induce significant abnormal head-cover vibrations in the frequency band 6–8fn (fn is the rotational frequency), restricting the operational range and hindering further increases in head. This practical engineering issue has rarely been reported and lacks effective solutions. To address this challenge, we innovatively added a thin horizontal blade to each of the guide-vanes to control rolling vortices. Systematic computational fluid dynamics (CFD) analyses were conducted to assess the influence of the horizontal blades on both the rolling vortices and the performance of the pump-turbine. Results indicate that the horizontal blades effectively segment the large rotating rolling vortices in the vaneless space into slender wall-attached vortices and significantly mitigate the abnormal pressure pulsations (6–8fn). In turbine mode, the horizontal blades reduce the circumferential velocity of the vaneless space water ring and decrease the backflow intensity at the runner inlet. This leads to an increase in flow capacity and power output, improves the S-shaped characteristic curves, and facilitates the rapid attenuation of runaway oscillations. Conversely, in pump mode, the horizontal blades exacerbate flow blockage in the guide-vane passages in the hump region, resulting in increased hydraulic losses and a slight reduction in the hump zone margin. Additionally, the decreased distance between the guide-vanes and the runner blades slightly amplifies the rotor-stator interaction pressure pulsations during normal operations. This modification of the guide-vane presents a promising strategy for enhancing operational stability in high-head pump-turbines and offers a novel design perspective for future advancements.